When a photon interacts with a material, an interaction occurs that causes its atoms to change their quantum state (a description of the physical properties of nature at the atomic level). The resulting state is called, precisely, photoexcitation. These photoexcitations are conventionally thought to kill each other when they come close together, radically limiting their density and mobility. This in turn limits how efficient devices that rely on photoexcitation such as solar cells and light emitting devices can be.
But in a study published June 19 in the journal Nature Chemistry, scientists at Northwestern University and Purdue University challenged this assumption with evidence that annihilation depends on the quantum phase relationship of photoexcitation. This means that sometimes photoexcitations do not annihilate each other when such quantum phases interfere destructively.
“Quantum interference is often believed to be fragile,” says Northwestern’s Roel Tempelaar. “This is an exciting new direction for the use of quantum interference made possible by detailed chemical control of molecular crystals. Our team advances this field by experimentally demonstrating control of annihilation by quantum interference, a principle previously predicted theoretically by one of the authors of this study. This contrasts with the current viewpoint that annihilation takes place as a ‘classical’ (non-quantum) process.
Tempelaar is an assistant professor of chemistry at the Weinberg College of Arts and Sciences. He is a member of the Center for Molecular Quantum Transduction at Northwestern.
The study, led by Tempelaar and Libai Huang at Purdue University, shows that quantum interference sensitively regulates photoexcited behavior. By adding different chemical side groups to identical molecules, the team made molecules of perylene diimide — an industrial dye — crystallize in unique ways with different motifs. The photoexcitations within each crystal differ greatly in their quantum phase relationship, which in turn results in an order of magnitude difference in their annihilation rates.
The team performed quantum chemical calculations to predict the differences in annihilation rates between molecular crystals and corroborated those estimates with spectroscopic measurements. The researchers took great care to decipher the spectroscopic contribution of the excitation mobility – which allows photoexcitations to meet one another – from the annihilation process itself. This is achieved by time-resolved spectroscopy-microscopy, which allows the degree of mobility to be determined, and control of the laser intensity, which allows the probability of annihilation to be varied.
The researchers hope their work can be used to create new devices such as photoexcited solar cells that have high density and high mobility. Such enhanced devices would require detailed control of the photoexcited quantum phase, which can be achieved by means of crystals with uniquely designed packaging motifs. Applications can range from optoelectronics to quantum information science.
“This research helps pave the way for the design of more advanced molecular materials by utilizing quantum interference as the main material,” said the Templars.